The use of turbochargers has been wide spread, in the US, on production Diesel engines since 1954 and on gasoline engines since the 1960s. Turbochargers are used for several reasons, e.g., to increase power density and thus reduce the size of engines for a given power rating, which helps vehicle dynamics in terms of weight and reduced aerodynamic frontal area. Turbocharging is used by engine and vehicle manufacturers to meet mandated emissions by increasing engine combustion efficiency with the desirable reduction in CO, CO2 and NOx emissions.
Where a normally aspirated engine loses the energy present in the exhaust, a turbocharged engine recovers this energy by using it to drive the turbine wheel of a turbocharger at very high RPM. The turbine wheel is mechanically connected to a compressor wheel (27), which is then driven to spin at the same RPM. Each stage, compressor and turbine, can be described as consisting of multiple components: inlet, shroud, impeller (or wheel), diffuser, volute, and outlet. The compressor wheel is mechanically connected to, and driven by, the turbine of the turbocharger. The compressor wheel, located within the compressor cover (2), draws air in axially, accelerates the air via the high rotational speed of the wheel, changing the direction of the air to a radial direction, and expelling the air radially with high kinetic energy in the form of high velocity into the diffuser section of the compressor stage. The function of the diffuser is to recover as much kinetic energy as possible by translating the high velocity of the air into pressure and temperature. The diffuser geometry is normally defined by the compressor backplate on one side and the diffuser geometry area of the compressor housing on the other side. The diffusion zone then feeds to the volute which collects the air from the diffuser. The function of the volute may be diffusing, constant velocity, or even accelerating, depending upon the design intent. The diffuser communicates with the compressor outlet.
Turbochargers are designed to operate at a particular target boost, and typically run into aerodynamic and material limitations when boost, or pressure ratio requirements, exceed certain levels. For example the natural, useable limit for single turbochargers is around 4.3 pressure ratio. To achieve greater boost, there are a number of options:
Where there is a material limitation, one option is to change the compressor impeller from aluminum to titanium, which raises both the temperature capability and low cycle fatigue (LCF) life of the compressor impeller.
Where the pressure ratio is not sufficient for the application, a method, employed in the industry, as depicted in FIG. 4, is to use vaned-diffusers (26) on the exit side of the compressor wheel (27). This has the negative effect of narrowing useable map width, combined with an elevation in temperature of the compressed air, the compressor wheel, and the vanes in the diffuser section. Exacerbation of both vane and impeller blade excitation, and thus high cycle fatigue (HCF) is a serious issue with the use of vaned-diffusers.
Another method commonly employed is a series compressor configuration. In this configuration, the discharge air from the first stage, or low pressure (LP), compressor is fed into the intake of the second stage, or high pressure (HP), compressor where the gases are once again compressed to produce higher pressure. In this configuration the air is often intercooled between stages for many reasons, some of which are to increase the air density, to improve the thermodynamics or to enable more cost effective materials to be used. The series configuration can take several forms. A regulated two stage turbocharger can have the compressor stages in a series configuration while the turbines are in either series, parallel, sequential, or regulated. The regulated configuration allows the turbines to be in either series or parallel configuration, depending on the position of a bypass valve.
Packaging of series or regulated turbochargers is especially difficult as the envelope used for the two turbos is usually expected to be that of a single turbo, in the already crowded under-hood environment. The turbine housings must be located in close proximity with each other as there are usually valves or bypasses controlling the turbine flow in both the specific turbocharger and its mate to influence turbine back pressures, flows, and compressor boost levels. Often in a regulated, or in a sequential, turbocharger configuration, there is one turbo with a small turbine housing and turbine wheel to provide good turbo response, and then a larger turbocharger, with large turbine housing and turbine wheel, to provide adequate boost to the high end of the operating range (high gas flow volumes). The compressors are generally in close proximity because the turbines are close to each other, and the compressor stages are mechanically connected to the turbine stages on each turbo. This close-coupling often causes problems with tight “U” bends of the compressor piping into the high pressure stage. Close coupling also exacerbates excitation of the downstream compressor blades. Any bypass or boost regulating valves have to also be incorporated into the package. Turbos in this configuration are usually stock turbos used in other applications to keep capital costs down and to minimize general part number count. With a prime rationale for turbocharging being to increase power density and thus reduce engine size, for improved vehicle aerodynamics, adding additional turbos is very challenging from a space perspective.
Turbochargers consist of a compressor wheel (or impeller) and housing, a turbine wheel and housing, and a bearing housing. These components, plus the thrust bearing, are configured for the turbocharger to operate in one direction of rotation only. To produce a turbocharger with an opposite direction of rotation, then each of these 6 components must be designed and manufactured specifically for the counter-rotation requirement and segregated from the remainder of production to prevent manufacturing and assembly errors. Considering also the need to remanufacture turbochargers at intervals, it is accepted practice within the business of turbocharger manufacture that it is more cost effective and more rational to have all turbocharger models rotate in the same direction. While directions of rotation will vary among different turbocharger manufacturers, they tend to keep to one rotation direction within their range of products.
Turbocharger performance is measured by several parameters including: (See FIG. 7 for a typical compressor map) Pressure ratio, Efficiency, and Map Width. These parameters are interlinked to a great extent so one parameter cannot be altered without affecting the other two.
Pressure ratio is the ratio of air pressure out of the compressor to the air pressure into the compressor (P2/P3), and is depicted on the Y-axis (55) of the map. For a compound or series turbo the total pressure ratio is the ratio of air pressure out of the high pressure turbo to the air pressure into the compressor of the low pressure turbo. The pressure ratio is depicted as the Y axis on a compressor map in FIG. 7.
The total-to-static (pressure) efficiency measurement of a compressor stage is the most representative method for representation of compressor efficiency. In its most simple form this is the ratio of the discharge pressure to inlet pressure, divided by the ratio of the discharge temperature to inlet temperature. The efficiency of the turbocharger is depicted on a compressor map as islands (74) of equal efficiency in the engine operating regimes of the map. Total-to-static efficiency is calculated by:
  η  =                              (                                    P              2                        /                          P              01                                )                                      (                          k              -              1                        )                    /          k                    -      1                      (                              T            2                    /                      T            01                          )            -      1      where P2/P01 is a measure of the isentropic work available to the order of specific heat ratio, and T02/T01 is a measure of the actual work done.
The map width of a typical turbocharger is depicted as the left and right boundaries of the map. The left boundary is the surge line (71) in FIG. 7. This is a test-generated line. At each speed line (73), the surge point is detected, noted, then interpolated for the entire map. At the surge point (depicted in FIG. 7 as the point on each surge line where the constant speed line (73 intersects the surge line (71)) oscillatory flow behavior causes a flow blockage. In the surge condition the flow detaches from the suction surface of the blade causing instability in the flow, which oscillates as the flow attaches and detaches from the blade. The surge condition moves with installation conditions so it must be tested for each set of installation parameters. In the surge condition the turbo reacts violently and must be kept out of this operating regime.
The right boundary is the choke line (75) in FIG. 7. This line is generated by selecting a minimum value of efficiency (often 65%), on each speed line in the region where there is a steep drop in efficiency caused by the air flow reaching sonic velocity. In the choke regime, the turbo operates smoothly but the pressure ratio (depicted at the Y-axis (55)) and efficiency fall, and temperatures rise.
The compressor map useable operating conditions can be made wider by several methods:
Adding a compressor recirculation feature, in slots or holes in the area of the compressor intake, delays the onset of surge to move the surge line to the left by increasing the effective volume flow rate through the inducer of the wheel to prevent stall on the blade. This increased stability range comes at a small efficiency loss but usually at a total increase in map width. Adding a recirculation feature also tends to force the onset of surge to be more abrupt. A huge downside to the recirculation bleed solution is often a very strident, high pressure, high amplitude noise, at the frequency of the number of full blades multiplied by the turbo speed in RPM (for a 7 full-bladed compressor wheel this frequency is 7N), 13 KHz for example. This problem often is resolved by solutions more complex than the execution of the recirculation flow strategy. U.S. Pat. No. 5,399,064 (Church et al) utilizes a type of muffler inserted into the recirculation cavity. Another patent, U.S. Pat. No. 6,623,239 (Sahay) utilizes a reflective cone to prevent the strident frequency from being transmitted through the inlet ducting.
Variable inlet guide vanes (IGVs) are used at the compressor inlet to change the angle at which the incoming air impinges on the rotating compressor impeller. In FIG. 8 the airflow (1) into the compressor is turned by the IGVs (81) housed in the compressor cover inlet such that the vectors of the inlet flow are changed to impart general rotation within the compressor cover, and thus the compressor wheel sees this rotation at its inlet. See U.S. Pat. No. 6,994,518 (Simon), U.S. Pat. No. 3,922,108 (Benisek), and U.S. Pat. No. 7 ,083,379 (Nikpour). These vanes can be manipulated to induce swirl in the incoming air such that it impinges on the rotating compressor wheel at an angle, either pro-rotation, or counter rotation. Using this technique, the useable regime in the operating envelope of the turbocharger map can be made to move to the left, or right of the static map. When the IGVs provide inlet swirl, counter to the direction of rotation of the compressor impeller, the entire surge line moves to the right, with a small movement of choke flow to the left, and an increase in pressure ratio. This decreases surge margin but results in the peak efficiency islands moving to higher mass flows or to the right on the compressor map. By being able to rotate the IGVs to produce flow the other direction (in the direction of rotation) surge margin is gained, a slight reduction (over the benchmark) in choke flow is seen, and pressure ratio is reduced as the amount of work done by the compressor decreases. A technical shortcoming of IGVs or pre-swirl vanes is that, in order to maximize flow for a compressor wheel, the compressor wheel blades are made as thin as possible, which makes them susceptible to high cycle fatigue (HCF) problems. A structure or blockage placed in front of the compressor wheel and inlet exacerbates any blade frequency condition which may be present with the result that much qualification work must be done to ensure that inlet guide vanes do not cause any increase in excitation of the compressor wheel blades. These excitations usually lead to compressor wheel HCF blade failures. Of course the same problems—blockage and excitation—which afflict compressor wheel blades also afflict the inlet guide vanes. Individual inlet guide vanes are themselves very expensive to manufacture because the “blockage” and “excitation” rationales force them to be quite elegant. Adjustable inlet guide vanes become very expensive as the controlling and operating mechanisms (82) must also be quite elegant and compact to meet the above criteria.